1. Field of the Invention
The functional properties of food proteins depend on their conformation in food systems. The relationship of protein structure to functionality is such that altering the chemistry of food proteins can improve functional properties such as solubility, viscosity, gelation, fat emulsification and foaming. The conversion of protein amide groups to carboxyl groups by deamidation improves solubility and other physical properties of proteins under mildly acidic conditions. Improving solubility, emulsifying or foaming properties of edible proteins enhance their use as functional ingredients in many food systems, including beverages, pourable and nonpourable dressings, whipped toppings, frozen desserts, confections, baked goods and meat.
An enzymatic approach to protein deamidation offers several advantages over a chemical approach, including the speed of reaction, the fact that the reactions take place under mild conditions such as neutral pH and room temperature and, most importantly, they are highly specific. The mild conditions reduce energy costs and the high specificity increases processing efficiency and minimize the need for downstream processing.
2. Description of the Prior Art
Enzymatic deamidation of food proteins has received little attention until recently. Gill et al., Irish J. Food Sci. Technol., vol. 9, (1985), p. 33, detected limited deamidating activity of Bacillus circulans peptidoglutaminase towards casein and whey protein hydrolysate. Motoki et al., Agric. Biol. Chem., vol. 50, (1986), p. 3025, used guinea pig transglutaminase to catalyze the hydrolysis of amide groups of glutamine residues in casein. Kato et al., J. Agric. Food Chem., vol. 35, (1987), p. 224, developed a method to deamidate food proteins by treatment with proteases at pH 10. They observed, however, a significant amount of uncontrolled proteolysis concurring with deamidation. Furthermore, processing of food proteins at pH 10 is undesirable for a variety of reasons including the implication of alkali-treated foods in causing kidney damage in rats [Woodard and Short, J. Nutr., vol. 103, (1973), p. 569].
Hamada et al., J. Food Sci., vol. 53, (1988), p. 671, used the peptidoglutaminase from B. circulans to deamidate soy peptides and proteins. Peptidoglutaminase (referred to as PGase also) readily deamidated soy peptides but its activity towards the intact protein was small. They suggested that limited deamidation was due to the large molecular size and/or unique conformation of soy protein. There is a need to provide for an efficient enzymatic deamidation process for food proteins of large molecular size and unique conformation at near physiological pH.